In the project we developed in Unity, if we want to work with UI Builder, UI Toolkit must be installed.

When the following error is received in UI Builder..

"Your Project is not configured to support UI Toolkit runtime UI. To enable runtime support, install the UI Toolkit package."

The reason for this error is that "UI Toolkit" is not installed.

The following steps must be followed to install the UI Toolkit..

• 1 - "Package Manager" opens.
• 2 - "Add" (+) is printed.
• 3 - Select "Add package from git URL…".
• 4 - "com.unity.ui" is written.
• 5 - Added with "Add".